Fix Flutter app build on Linux

[thiagokisaki]

This pull request fixes Flutter app build on Linux. Fix #1381.

The execute_process calls in CMakeLists.txt were broken. Neither dart run realm install nor dart run realm metrics were succeeding. Instead, they were returning Could not find file 'realm'.

Realm binaries were not being installed, thus provoking the error during the build.

This was solved by providing WORKING_DIRECTORY option to execute_process.

[nielsenko]

Thx 👍

[nirinchev]

@cla-bot check

[cla-bot]

Realm welcomes all contributions! The only requirement we have is that, like many other projects, we need to have a Contributor License Agreement (CLA) in place before we can accept any external code. Our own CLA is a modified version of the Apache Software Foundation’s CLA. Our records show that CLA has not been signed by @thiagokisaki. Please submit your CLA electronically using our Google form so we can accept your submissions. After signing the CLA you can recheck this PR with a @cla-bot check comment. The GitHub usernames you file there will need to match that of your Pull Requests. If you have any questions or cannot file the CLA electronically, make a comment here and we will be happy to help you out.

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[thiagokisaki]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[nirinchev]

Thanks for your contribution, @thiagokisaki 🎉